The concept of a mobile communication system is that a large number of base stations, i.e. radio access nodes, cover a geographical area and provides mobility to a number of users equipped with wireless devices, which may be denoted e.g. User Equipment, UEs or mobile handsets. Herein, mainly the term UE will be used. A very basic requirement of such a system is that, as the wireless devices move from one cell to another, it must be possible to hand over an ongoing call or data session from one base station to another in a manner which does not cause annoying disruptions in the calls or data sessions.
Mobility in 3GPP Networks
The purpose of the handover, or Hand Over, HO, is to manage the mobility of UEs from the coverage of one radio access node to another. This ensures that the UE is being served by the “best”, or most suitable, cell at all times, and thus avoids connection releases as the UE moves out of a coverage zone of one cell into the coverage zone of another. A HO can also be performed for capacity reasons, e.g. in case of load balancing when neighboring cells have very different loads. In that case, what motivates a HO is the optimization of the overall system capacity and/or end user quality of experience, QoE.
In 3GPP systems, HO is network controlled based upon UE measurement reports of the radio quality level of the serving cell and neighboring cells, which may be potential “next serving cells”. These reports are configured via measurement configuration procedures.
The mobility framework in 3GPP networks for UEs in connected mode is based on the concept of so-called mobility events. The UE is configured to perform periodic measurements of Reference Signal Received Power, RSRP, and Reference Signal Received Quality, RSRQ, based on radio signals received from the serving cell and from adjacent cells. Mobility events are configured in such a way that, when a predefined condition related to these measurements is fulfilled, the UE sends a measurement report indicating the occurred event.
Intra-LTE HO
The Intra-LTE HO feature configures a mobility event called EventA3 as defined in 3GPP TS 36.331. The EventA3 implies that one or several neighbor cells become better, by an offset, than the serving cell. An intra-LTE HO preparation phase is illustrated in FIG. 1.
The process employed by the UE for the evaluation of surrounding cells uses parameters sent by the serving eNB to the UE. These parameters include hysteresis and offset values, time to trigger, and optionally cell individual offset margins.
The Intra-LTE HO feature is based on the evaluations reported to the eNB by the UE. The serving eNB uses the reports to select and prepare the target eNB, then ultimately execute the HO.
Measurement Configuration
Once the UE is configured, during connection establishment, measurements commences on the serving and neighboring cells when the RSRP of the serving cell falls below the value defined in the sMeasure parameter. The UE detects neighboring cells through intra-frequency searches. The UE uses either RSRP or RSRQ measurements to determine whether the EventA3 condition is fulfilled. Measurements of RSRP or RSRQ are performed on the serving and detected neighboring cells.
The UE then uses an offset value, a3offset, a hysteresis value, hysteresisA3, and a time to trigger, timeToTriggerA3, to determine whether to trigger the EventA3. The parameter cellIndividualOffsetEUtran can be set per cell relationship and the value is added by the UE to the measured value of the neighbor before EventA3 evaluation is done by the UE. The UE uses the following formula for evaluating entry to EventA3:Mn+cellIndividualOffsetEUtran−hyteresysA3>Ms+a3offsetwhere Mn=Measured value of the neighboring cell (RSRP or RSRQ) and Ms=Measured value of the serving cell (RSRP or RSRQ).
Once entering EventA3, the UE continues to evaluate EventA3 a predetermined time (timeToTriggerA3) before the event is fulfilled and a measurement report is sent to the serving eNB. These measurement reports contain measurements for the serving cell and a configurable amount of detected intra-frequency neighbor cells. Measurement reports are sent periodically while the EventA3 condition is fulfilled.
The measurement reports are event-triggered and resent periodically as long as the event is fulfilled. The hysteresis value is used to avoid immediate retriggering of the event.
HO Evaluation and Execution
The eNB starts the so-called “best cell evaluation process” when a measurement report is received from a UE. The process performs selection of a target cell for the UE and HO preparation.
When an EventA3 measurement report is received for a UE, the serving eNB sends a request to the target eNB about resources to facilitate the HO. Upon receiving a positive acknowledgement to the HO request, the serving eNB sends HO directions to the UE and suspends scheduling new data transmissions to the UE.
The UE starts the move when the source eNB sends an RRCConnectionReconfiguration message to the UE telling it to perform a HO. After successfully accessing the target cell the UE sends an RRCConnectionReconfigurationComplete message. The time between these messages is the handover interruption time, which according to earlier measurements in an LTE network is on average 90 ms. However, some of the HOs have a significantly larger outage time, which may be above 200 ms.
Conversational Speech and VoIP
Voice calls are a part of the traffic in mobile communication systems. Below, the characteristics of conversational speech, i.e. inter-human verbal communication, will be described. The reason for this will be apparent further below.
Conversational Speech Characteristics
There exist several models for conversational speech. The models aim to correctly generate the on-off patterns in conversational speech, including short silence gaps and the effects of interaction between parties. One such model, called the Brady model, is illustrated in FIG. 2.
Speech can be modeled as short talk spurts, separated by silence gaps. These silence gaps can be classified into three types. The first type of silence is when one person is listening to the other person in a conversation. The second type of silence is between sentences where the speaker takes a longer break. The third type of silence is between words in a sentence.
For silence periods between sentences, the duration is between 200 ms and 2 seconds, with median duration 500 ms. For silence periods between words, the duration is between 0 and 200 ms, with median duration 30 ms. The length of a talk spurt is between 5 ms and 5 s.
According to a two-way conversational model presented by the creator of the Brady model, the average percentage of the mutual silence state in a conversation is about 19% and the average duration of a mutual silence is about 300 ms.
VoIP Traffic Characteristics
Voice over IP, VoIP, represents the transmission of the conversational speech service over Internet Protocol, IP-based networks. It has been widely applied in both wired and wireless communications.
A large variety of technologies that support VoIP services have been developed. From a protocol stack perspective, the VoIP protocols are built on top of either the Transmission Control Protocol, TCP, or the User Datagram Protocol, UDP, as shown in FIG. 3.
The traffic characteristics of the VoIP packets delivered to the underlying network protocols are to a large extent determined by the compression/decompression applied to convert audio signals into a digital bit stream and vice versa. The traffic characteristics include silence duration due to the on-off conversational characteristics described above, voice payload size, packets inter-arrival time, etc.
The traffic characteristics of lower protocol layer packets depend on the overheads, structure and implementation of the underlining protocols. Nevertheless, they should be similar to the VoIP packet characteristics.
Traffic Characteristics During Silent Periods
As stated in above, conversational speech traffic has silent states due to the nature of human conversation. VoIP technologies utilize this property in order to reduce bandwidth usage. Two categories of solutions have been developed to address the transmission adaptation to the silent periods.
A. Silence Suppression
When silence suppression is applied in a coder/decoder the packet sending rate is reduced during a silent period or state, i.e. when the user is not talking. The silent state is detected by voice activity detection, VAD, which is implemented in the transmitter. In general, a VAD algorithm requires about 200 ms to detect the silence period. Among the three types of silence gaps described above, the first two types, i.e. silence while listening and the inter-sentence silence, can be detected by the VAD.
Once a silence period is detected, a Silence Insertion Descriptor, SID frame may be sent to the receiver. The SID indicates the beginning of a silence period and provides a noise level for the receiver to configure parameters for generation of so-called Comfort Noise. The transmission of SID frames during the silence period may be either triggered by a significant change in background noise, or sent repeatedly with a periodicity of e.g. up to 480 ms.
For the detection of a silence period when applying SID frames, a period may be deemed to be a silence period e.g. if there are no packets received during a time period longer than a threshold of 100 ms; otherwise, it may be considered a speech period.
B. No Silence Suppression
Some VoIP applications, such as Skype, do not employ silence suppression. That is, silence is not handled differently than active speech, but is encoded and transmitted to a peer. This increases the bandwidth usage as compared to the case when silence suppression is used. However, transmitting these silence packets has some advantages, as for example, it maintains the UDP bindings at Network Address Translation, NAT. In the case where media traffic flows over TCP, use of silence packets avoid the drop in TCP congestion window size, which otherwise would take some Round-Trip delay Time, RTT, to reach the maximum level again. However, the packet size during a silence period may still decrease.
In order to eliminate packet losses during LTE HOs, packet forwarding has been implemented. That is, packets arriving to the former serving cell are forwarded to the new serving cell. However, packet forwarding cannot solve the problem that the packets are delayed at least a time equal to the handover outage time, i.e. the time when the UE is disconnected from the former serving cell and not yet connected to the new serving cell. For a VoIP application, packets that arrive too late to be useful are just as bad as real packet losses.
Further, it is known that the delay over the so-called X2 interface between eNBs may be significant. Depending on deployment, forwarding over the X2 interface can add an additional ˜50 ms delay to the forwarded packets, which degrades the VoIP quality.
The solution described herein aims to reduce the VoIP QoE degradation associated with HO, an objective typically neglected by other mobility algorithms.
In the prior art, one example has been found of that the properties of conversational speech have been exploited in association with HO, namely [1]. In [1], the state of a voice call is determined. If the state is silent, or idle, the HO is executed, and if the state is active, the HO is delayed in wait of that an idle state may be entered. If no such state is entered within a certain time, the HO is executed in spite of that the voice call is in an active state. The analysis of the voice call in [1] is performed in a handset.
However, the inventors have realized that even what is described in [1] is associated with problems, and that there is a need for an improved HO mechanism taking QoE into account.